text:
What is the margin of difference between copying and developing new?

Iran Flaunts New Missile and Jet Engine Technology

The above article says that Iran is able to make exact copies of American General Electric J85 and Williams FJ33/FJ44 jet engines.
Does this mean that they are able to design and develop a jet engine from scratch?
If NO, why?

If they were able to develop from scratch, then why make exact copies?

Sure there was a prior answer, but only found the related https://aviation.stackexchange.com/a/49827/60886

This certainly does not mean they are able to design and develop usefull jet engines from scratch.
This pdf-file from rand.org offers some insight to the projected and actual cost of designing jet engines, ranging form 100 million to 1.5 billion dollars. While cost is not the subject of this question, it is an extremely good indicator of the difficulty of the process. The paper is dated october 1982, and one can be certain that designing a jet engine from scratch has not become cheaper or easier over the years.
The difficulty in designing modern jet engines largely lies in metallurgy, and having high level of understanding of the "inner workings" of the engines, based on research data of actual engines. Computational fluid dynamics can never fully replace actual empirical data (there is a good question with good answers about that specific subject here, I'll try to find and link it). it takes decades to accumulate this data and build the understanding.
To copy a GE J85 jet engine form the 60's is not a miraculous achievement, I personally would not "flaunt" that. The copying of the FJ33/FJ44 is not all that wonderful either, it is not excatly the pinnacle of high tech. What also remains uncertain is whether either of these is actually a reliable unit, or even functional for that matter.
In fact, pretty much anyone can design and build a jet engine:

 /

The real question is, how functional the final product is from efficiency and reliability points of view.

"one can be certain that designing a jet engine from scratch has not become cheaper or easier over the years" – But dollars have, so the numbers would be even higher in 2022 money.

Absolutely, the multiplier is roughly 3, so in todays dollars the top end of the spectrum would be about 4.5 billion. Then again, methods have improved, so the same relative technological progress would not be as expensive.


text:
Do jet fighters use Turbojet engines, and do cargo/passenger aircraft use Turbofan engines?
I have a feeling that Turbofan engines are used in jet fighters and Turbojet engines are used in cargo or passenger aircraft.
Is my perception correct?
Do jet fighters use Turbojet engines?
Do cargo/passenger aircraft use Turbofan engines?

would this answer be helpful?

No
Turbojets were mainly used in the Fourties and Fifties. As soon as pressure ratios had become high enough to allow adding a fan stage, turbofans replaced them. And the distinction is less between civilian and military engines but between low-bypass-ratio and high-bypass-ratio.
Civilian jets which used the same engines as military ones did so because there was not much else available: From the de Havilland Ghost in the Comet and the de Havilland Venom to the GE J79 in the Convair 880 and the F-104 Starfighter or the B-58 Hustler (in the two latter ones with an added afterburner) both sides shared the same engines.
For the rest of the story I refer you to this answer.

Pretty much all "jet" aircraft designed since the 1960s (if not the late 1950s) have used turbofan engines, and bypass ratios have tended to increase over that time frame, as higher bypass tends to produce more thrust for a given specific fuel consumption.
The Boeing 707 and older versions of the B-52 bomber (both introduced close to 1960) are examples of aircraft designs with turbojets -- but pretty nearly everything newer, including fighters as well as bombers and transport aircraft, have used turbofan power.

Concorde's Olympus was an exception, though.

@Zeus When Concorde was designed (first flown 1969) turbofans weren't quite ready for supercruise.  By the time it entered commercial service (1976), they were just about getting there.  Supersonic was the last bastion of the no-bypass turbojet.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


text:
What is more correct: Mass or Weight and Balance?
When referring to mass and balance calculations, is it better to refer to mass or weight and balance. Or does it even matter? Are both okay?

Mass and balance is more accurate, it is used e.g. by SKYbrary (Eurocontrol). However the most common wording is weight and balance, it is used e.g. by Airbus.
ATPL schools use mass and balance, e.g. Jeppesen or Mermoz Institute.
The center of mass is the barycenter of masses, the center of gravity is the barycenter of weights. When gravity is uniform across the body volume these two centers are coincident. Thus centering the masses or the weights is equivalent.
When performing the centering, we manipulate mass values expressed in kg or lb, not weight values in N.

(Source).
A mass (kg) under the effect of gravity creates a weight force (N), which associated with an arm (m) creates a moment/torque (Nm) acting on the center of mass. Mass and balance is the check of both the total mass and how these moments cancel each other. However this centering is also valid when the masses are subject to additional accelerations due to turbulence or maneuvers, so the centering is not limited to moments created by gravity, this is a good reason for not talking about weight and balance.
Likewise when we says the maximum takeoff weight of the A320 is 68000 kg, we are actually using a mass unit and expressing a mass, not a weight.

It is largely semantics. I have never heard of someone using the phrase 'mass and balance' when referring to calculations about an aircraft.
At the same time, an important role in developing an aircraft is that of 'Mass Properties Engineer'.  Their professional society is SAWE, the Society of Allied Weight Engineers (historically, they were the Society of Aeronautical Weight Engineers).  From their website:

The Society of Allied Weight Engineers (SAWE) is an international, non-profit organization of mass properties engineers dedicated to promoting the mass properties discipline. Our members work in civil and military aircraft, space and missile systems, marine and land vehicles, on-shore and off-shore platforms, and other industries that support our efforts.  If you are looking for help predicting, controlling, and verifying the weight and balance of your products, you've come to the right place!

The SAWE website links to a presentation explaining their role in the world, they switch between both terms depending on context.

It's somewhat of a false dichotomy. You are balancing weight, but you're balancing it by shifting mass.
If Earth had 0 gravity, you'd still be balancing mass, to perform and survive high-g maneuvers. But if a large spacecraft had to fly so close to a black hole that gravity varied across the fuselage, you'd definitely be balancing weight.

What is a high-g maneuver with 0 gravity? Infinite acceleration? ;)

@Chris Earth's gravity has no effect on the g-force you get in sharp turns. Fighters would still have to hit 7-9g to stay competitive, although they wouldn't call it "g".

@Therac I think what Chris meant is that if g is 0, you would need infinite acceleration to reach 7-9g's. Because 'g' is a measure for how many times you experience earth's gravitation. So from that perspective, earth's gravity definitely does have an effect on the math. (But indeed not on what the fighter would experience)

@Opifex I get the joke now. Though a "high-g maneuver" is essentially a single term by now.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Can you cross an ADIZ without filing a flight plan, if departure and arrival are in the US?
I am planning a flight from Houston to Florida. The quickest way for me is to just fly direct to across the water saving many miles. I am trying to do this VFR. Is this possible without filing a flight plan and under VFR flight following or is there a different procedure to follow?

Related - https://aviation.stackexchange.com/questions/40941/what-happens-if-i-cross-the-adiz-line-without-a-flight-plan-and-no-clearance-or

I appreciate the question in the spirit of general knowledge/technical ability/etc, but how much effort would it be to file a flight plan?

@randomhead True ahaha! So filing a normal flight plan would impose no problems including having to undergo Customs?

I don't have any knowledge on that topic at all, but my gut reaction is that Customs procedures apply (or not) independently of whether you file a flight plan (or not). Provided you don't stop anywhere outside the US, I don't see why Customs needs to get involved at all. Of course the Customs people may have another opinion; I'm not an expert.

Re "Is this possible without filing a flight plan" - why would it not be possible?  Lift drag etc don't care about flight plans.

I did the flight from New Orleans area direct to Florida, crossing the ADIZ under VFR Flight following, there were no issues whatsoever, as we were constantly identified. In some areas we lost reception at around FL100. Other Airplanes flying higher were instructed to relay messages. This Flight allowed us to profit from the nice 40kts tailwind. So indeed a flight plan is always a good option, but it was possible under flight following. As we did not officialy leave the US there were no customs involved.

Thanks for coming back to share the answer :)

From the FAA:  (the first hit on a Google search...)

Air Defense Identification Zone (ADIZ) All aircraft entering U.S.
domestic airspace from points outside must provide for identification
prior to entry or exit. ADIZs have been established to assist in early
identification of aircraft in the vicinity of international U.S.
airspace boundaries (AIM Section 6, 5-6-1).
Many aircraft inbound to the U.S. will cross an ADIZ. There is no ADIZ
between the U.S. and Canada. According to FAR Part 99, if
penetrating an ADIZ, all aircraft of U.S. or foreign registry must
file, activate, and close a flight plan with the appropriate
aeronautical facility. In addition to normal ADIZ position reports,
and any other reports Air Traffic Control may require, a foreign civil
aircraft must give a position report at least one hour before ADIZ
penetration, if it is not more than two hours average cruising speed
from the U.S.
For Defense VFR (DVFR) flights, the estimated time of ADIZ penetration
must be filed with the appropriate aeronautical facility at least 15
minutes before penetration, except for flights in the Alaskan ADIZ, in
which case, report prior to penetration. Additionally, VFR pilots must
receive and transmit a discrete transponder code.
Be sure to activate your flight plan before crossing the ADIZ.
ICAO VFR flight plans must include in the transmitted line 18 "other
information" section: DVFR/estimated United States ADIZ penetration at
time (UTC) and estimated point of penetration (latitude/longitude or
fix-radial-distance).


text:
What is the best way to determine wind correction angle?
As a new instrument student, I would like to know the best and efficient way to determine a rough wind correction angle. Calculating it with help of the e6b or other ways is often good to do before flight, but is not practical to do in flight, especially when dealing with various other tasks. As the wind is constantly changing in direction and strength, I am wondering if there might be a good rule of thumb or other tricks one might use to determine a rough WCA. Thanks!

Closely related, maybe a dupe?

I made it through Instrument training without needing to calculate a WCA in the air, just trial and error to figure out where I needed to point the nose to keep the needle centered.

There are 2 methods to use whilst in-flight. Both start with a "max drift" calculation which is pretty easy to do:
$$\text{max drift} =  {60 \over \text{TAS}} \times \text{wind speed}$$
Therefore, for the most part, in many typical GA aircraft you're looking at about 2/3 the windspeed.

I prefer the simpler "clock" method.
Now the "clock" part is that you determine in-flight what the wind direction is with reference to your heading.

30 degrees - "half hour" - half the WCA
45 degrees - "three quarters of an hour" - three quarters of the WCA
60 degrees - "a whole hour" - all the WCA

The other method uses your Direction Indicator to visualise the crosswind component. It also has the advantage of being able to use a similar method to visualise the head/tailwind component too.
It is very well illustrated in this article

Ditch the E6B and get a Jeppessen CR3 circular computer.  The wind correction computer on the back uses concentric circular discs only, no slidey bit, so it's easy to manipulate with one hand.  You'll have to locate your pencil dot that you use to mark the wind direction and speed on the wind disc in your mind if you're only using one hand, but you can get pretty close that way.
Practice with it and you'll find you can hold the wind side up, spin the discs with your thumb, and eyeball a wind solution, based on your mental placement of the "wind mark", in about 10 seconds.
Spend some time messing around with it, working out solutions for a range of winds and directions, say, 30, 60 an 90 degrees off the nose or tail, and wind speeds of 10, 20 and 30 kts, at your cruise speed, and pretty soon you will be able to make instant ballpark guesses for drift correction and groundspeed that are good enough, unless you are trying to hit a small island in the middle of the Pacific.

still not really practical in flight if you're an inexperienced instrument pilot. You forget just how overwhelming it can be.

That was the point of my last paragraph.  I can just make a mental image of the wind dial and make a close enough guess.  Main thing is get rid of the E6B for a proper professional circular computer.


text:
How does a jetstream boost an aircraft's speed?
Jetstreams are streams of high-moving west-to-east winds (200mph to 300mph, source) which most pilots are trained to use to cut down on their fuel costs (source). In the same linked article, it mentions private jets almost always fly at maximum speed anyway, even when on a jetstream.
Assuming a pilot maintains the same airspeed before and after entering a jetstream, how much would be the expected change in groundspeed in practice? Is it just the airspeed plus the tailwind?
I imagine it depends on the type of the aircraft, so consider only a typical commercial airplane (Boeing, Airbus? I'm not sure which one is more typical).
I'm just curious in knowing how the plane's speed is affected and why it changes that way.

Welcome to aviation.SE! We already have a lot of questions on airspeed, ground speed and wind, this question or this one could be a good starting point.

@Pondlife Thanks! Feel free to mark this one as a duplicate to either one. And thanks for the links to both :)

It works like you have assumed: Just add velocity vectors (velocity of aircraft relative to air and wind velocity relative to ground). There is no maximum ground speed, only maximum airspeed. As jet streams don't change aircraft airspeed, a pilot takes full advantage of them, whatever its current groundspeed. It's also the same than a person walking in a moving train, there is not effect of the train speed on the person (in non-relativist physics), but the ground speed is the sum of velocity vectors.

The difference between private jets and commercial, is privates want to get there are soon as they can, whereas commercials are on a schedule. It benefits the airline more to throttle back in a tail wind to maintain the arrival time, and save fuel, than it does to get there early.

Just to clarify the OP's referenced speeds... The jetstream in the thermosphere that moves at 200-300MPH is not the same jetstream that aircraft fly in.  The lower Jetstream (at about 30,000ft) that aircraft use only flows along at about 100MPH avg. The official definition of the jetstream starts at 50kts, and 50-150 is what is normally observed.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
How much more efficient is an aircraft with no windows?
How much more efficient is an aircraft with no windows (in terms of operation)? Consider the hypothetical situation where you have two aircraft identical in every way, except one of them has no passenger windows. Or is the difference so small that its negligible?

Technically, not a duplicate because the premise of the question is different. This question asks if the aircraft are identical in every way except for windows. One could presume that the identical comparison aircraft has window-shaped cutouts with window-sized opaque material installed.
However, given that premise, the question does not make any sense. If the comparison aircraft has exactly the same cutouts for windows, empty weight, and drag associated with these structures then the fact that some "windows" are not transparent makes absolutely no difference in performance.

@RudyB Why are you philosophizing this, isn't it self-evident that "has no windows" means a fuselage without holes!?

@RudyB a window is a window regardless of the material (or lack of) used to fill the void. I think the question means if there were no window related cutouts or reinforcements, otherwise all you’re changing is the material the window is made from, which is purely a weight change question. I think this question is a combination of the two now flagged as dupe.

No surprise this question was closed as duplicate instantaneously... however there is more to say about windowless aircraft than drag. Airbus is currently thinking of this with "Transpose" for the future. This is a serious concept, contrary to the CPI nth "revolutionary concept".

Many aircraft have "windows" that are not functional (for example in the area where on the interior there is a lavatory). A fuselage without holes is a very different issue, and cannot possibly be "identical in every way" as the original question asked, to a fuselage with windows. There are issues of rigidity of the structure, strength, drag, etc. There are manufacturing issues regarding cost and efficiency. Plenty of cargo vans have sheet metal "windows" because the window frames are stamped into the parts whether or not those frames will have glass placed inside them in the end.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Why aren't rudders automatic?
Why aren't rudder pedals automatic or connected with ailerons to keep the nose of the aircraft coordinated with the turn its entering? Wouldn't this make for an easier and smoother to operate aircraft?

Closely related

Related: Do modern aircraft require rudder input in order to perform a coordinated turn?

Does this answer your question? What's the purpose of yawing?

IMHO, this is a duplicate, in that knowing why one needs to control yaw also explains why linking rudder and aileron is not something you want in every airplane. The answer to this question is also found in the Q&A mentioned above, as well as https://aviation.stackexchange.com/questions/24051/can-perfect-turn-coordination-automation-be-achieved-by-linking-rudder-to-ailero, https://aviation.stackexchange.com/questions/25699/why-dont-all-ga-aircraft-have-yaw-dampers, and https://aviation.stackexchange.com/questions/27823/why-is-using-opposite-rudder-and-aileron-necessary-in-left-turn-for-coordin

The answers, all described in the duplicate and related posts, come down to: 1) it's not feasible to include such a feature in all planes, especially smaller/less-expensive ones, 2) it's not desirable to always remain coordinated, and 3) coordination in an airplane involves more than just a direct, proportional input of rudder along with aileron (this is closely related to the first point)

Also related: https://aviation.stackexchange.com/questions/3864/did-wolfgang-langewiesche-ever-change-his-mind-about-rudder-pedals.

Re "Why aren't rudder pedals automatic or connected with ailerons" -- Well, some are-- e.g. Piper Tri Pacer (connected to ailerons w/ bungees or springs), Ercoupe (no separate rudder pedals at all)

@quiet flyer: That makes me wonder how you steer an Ercoupe when taxiing.

@jamesqf-- nosewheel steering is connected to yoke, I believe.

@quiet flyer: That must be fun when landing & taxiing in strong crosswinds.


text:
What are the degree markers on the tail of this MD-10 for?
I took this picture of of Orbis's McDonnell Douglas MD-10-30. Tail: N330AU.

What purpose do the 0 and 2 degree markers serve?

It is there so the pilots can check the stab trim via the wingtip mirrors.

This tells the maintenance mechanics what trim state the elevator is in without having to run the pitch trim motor back and forth from the cockpit.

OK, call it the stabilizer. mechanic doesn't know the status of pitch trim. climbs into the cockpit, runs the pitch trim to the nose up limit, goes back, measures the pitch angle, runs back to cockpit, runs the pitch trim to its nose down limit, goes back and measures the pitch angle. Or (s)he looks at the index mark on the fuselage.


text:
Should you line-up the runway 90 degrees or follow the yellow marking?
I had a discussion with somebody recently. I prefer to do a 90 degree line-up on the runway, ensuring max take-off run available. (in GA airplanes...) However, somebody pointed out that one should follow the yellow line, which I disagree with. Aren´t they only to vacate the runway? Are there any official guidelines or truth to this? Thanks in advance!

Whenever anybody says you "should" do something that you believe to be pilot technique, you should challenge them to provide a reference...

@MichaelHall Absolutely! Was just weird coming from a flight examiner ...

DPE's aren't immune from being challenged to substantiate their claims!  I have read some crazy stuff some have allegedly claimed...  Recommend tactfully, and maybe after they sign you off...  ;)

Here's a commercial aviation response, but almost certainly applicable to general aviation, too.
Take-off distances are commonly measured from the threshold or, for intersections, the downwind edge of the taxiway (note that might actually depend on local regulations - might be worth checking the AIP). In any case, to achieve certified take-off performance, you need to replicate the geometric relation between main gear and runway threshold as closely as possible, which in essence most probably means a minimum distance line-up most of the time.
If you operate on runways much longer than your performance limitations, it doesn't really matter in practice, but that doesn't change the theory.
See e.g. the referenced document from Airbus publishing takeoff performance corrections for non-standard line-ups, and the baseline is a 90⁰ entry with minimum distance, ignoring any nicely curved yellow lines:
Airbus document
Having said all that, this is the performance engineer's view. I am not certain as to whether there's anything anywhere in the regs (however, if there was, it would have been commonly ignored and actively trained against in all airlines I used to work for).

Places I used to operate from were generally quite busy (GA and otherwise). That necessitated taking into account the often long queue of planes behind you, so the lineup was done in about 60 degrees to avoid blasting planes behind you with propwash when doing the run-up. On the rare occasions one was the only plane taking off, and in places where run-up at RWY lineup was not allowed, I was taught to obey the lines. "They don't paint them on the tarmac just for sh_ts and giggles."

-1 because what? I've presented two actual real world examples of CFI approved ops.


text:
Can the moment coefficient at the quarter chord point be negative?
After a calculation I obtained a value of -0.318 and I am just checking to see if this is reasonable.
An explanation for the consequences of having a negative value here would also be appreciated if possible.

Why don't you show your math and explain more clearly what you are trying to calculate.

I tried to give a somewhat general answer below .. I guess for clarification, some more details would be helpful .. how did you define your coordinate system, maybe a sketch?

Positive or negative here depends on the coordinate system you have used. An airfoil producing positive lift (up) will invoke a positive circulation, and that will require a counter-acting torque, which is then negative (pitch-down).
Some more detailed explanation can be found on wikipedia.
https://en.wikipedia.org/wiki/Pitching_moment
https://en.wikipedia.org/wiki/Aerodynamic_center
(I haven't checked either for correctness).
Furthermore, the CM (torque coefficient) is usually a function of the angle of attack - so - provide a simple example - on a symmetrical profile, running negative angles of attack, there will be a positive CM. (Aerobatic planes eg use that sometimes)


text:
What is the 'optimal' taper ratio?
I mean the optimal taper ratio for an unswept trapezoidal wing for minimising induced drag.
I see a lot of places recommending a value of 0.45 to give as close to an elliptical distribution, therefore maximising Oswald efficiency (to 1). However, Hoerner (Fluid-Dynamic Drag, chapter 7-2) has this plot which suggests the optimal taper ratio is about 0.356 (graphical line fit).

Similar graphs are produced in this paper: Induced Drag of High-Aspect Ratio Wings
Tommy M. Chen and Joseph Katz
This exact taper ratio is used by BAe regional jets also.
I haven't read about this thoroughly enough. Maybe this value considers root chord and wing area to be fixed, therefore as taper ratio is decreased the aspect ratio must increase to maintain a constant area. The increased AR (really the increased span) reduces the induced drag.
Does anyone know the answer? I'd really like to be able to justify my taper ratio for an aircraft design assignment.

The excerpt you posted mentions a further modification of the Oswald efficiency/taper ratio curve which is not applied to the graph in the excerpt. With that modification (which I believe is included in the following pages of the book), the optimum point may move to 0,45. For an assignment, the other places you found 0,45 from and the BAe wings are sufficient motivations to set it at 0,45 IMHO.
I'd like to mention that taper ratio is not the only way to adjust spanwise lift distribution, you can change the airfoils (which you mentioned you have excluded from your design) or you can add twist to the wing so that the wingtips work at a lower AoA. This has the added benefit of making the wing root stall before the wingtips where the ailerons are located.

Besides loss of authority from the ailerons in a stall, a tip stall is likely to induce a spin, which a root stall will not. ‘Wash-out’ towards the wingtips is desirable as a safety feature as well as reducing parasitic drag.


text:
What causes lift in aerofoil: Bernouli's principle or Newton's third law?
Reading materials online, I observe some explanation based on Bernoulli's principle, quoting the difference of air pressure above/below and difference of speed of airflow. While some say's Bernoulli's principle works only in special conditions (laminar, non-viscous, non-compressible, streamline, etc.) and the correct working is based on Newton's third law whereby, the airfoil pushes the stream downward and experiences lift upwards.
Bernouli's principle is also said to not work for symmetrical aerofoils and inverted aerofoil.
What is the final verdict of what causes lift?

Could you share your sources for the statements that Bernouli's principle only works in special conditions?

@GdD - https://www.khanacademy.org/science/physics/fluids/fluid-dynamics/a/what-is-bernoullis-equation It is pretty standard thing that bernouli's principle is valid only for streamline, non-turbulent, non-viscous, incompressible flows.

@RajArjit, actually, Bernoulli's equation is just formulation of conservation of energy for adiabatic fluid flows, and often given for incompressible flows only (then you don't need to add the equation for adiabatic compression), and as such is always valid.

… of course with turbulent flow you have to take the $v$ in the Bernoulli's equation to mean the average magnitude of velocity of the particles, which is higher than their average forward velocity. But that does not make Bernoulli's equation invalid, just more complicated.

I think it is erroneous to say that lift is solely produced just because of the Bernoulli's or Newton's law.
Sixty Symbols(A youtube channel) explains the best how a wing actually produces lift.

Instead of explaining I thought this video might help you more than my explanation. And also look at "Incorrect Lift theory" by NASA

The discussion of the isobars close together at the leading edge contributing to the "acceleration of air over the top" reminded me a bit of a jet engine.  The thrust indirectly is the energy source, as the movement creates this zone of increased pressure ahead of the wing.  This adds to the decreased pressure behind the wing.  Air moves from high to low. Wings work best when they have a slightly positive AOA (even when inverted!).  Sorry this was labelled "duplicate", as it adds to the understanding of a very important fundamental subject. +1.


text:
Helicopter Flight: What is the reason for decrease in Induced drag with increase in speed?
As per the figure the induced drag will decrease only if the AoA of the blade is decreased — But in the forward flight, cyclic input increases and decreases the AoA throughout the blade rotation cycle.
Also, in forward flight, a component of the Lift force is used to compensate for the parasite drag, so the overall Lift vector will increase. This should increase the induced drag further.
Where I am getting wrong? What causes Induced drag to decrease with an increase in speed?

As you mentioned, the main rotor thrust must support the weight of the helicopter in hover, and must also provide a propulsive force to overcome drag in forward flight.  However, the induced drag is also a function of how the air is deflected downward by the rotor.  The velocity of this downward deflection of air at the rotor is often called induced velocity.  This induced velocity is a function of not just thrust, but also the helicopter forward speed, rate of climb, air density, height above ground (when near the ground it decreases, a phenomena known as ground effect), etc.
Relevant to your question is how the induced velocity decreases as the helicopter goes from hover to moderate forward speed (even though thrust increases).  At hover, the rotor is sitting in its wake with a large induced velocity.  With some speed, the helicopter is moving away from it's wake, resulting in a smaller induced velocity.  This smaller induced velocity increases the angle of the incoming air to the blade, and therefore decreases induced drag (and even overall power required).

Thank you! It appears so simple, now!

From your question, which is perfectly logical, there is one aspect that is leading you to an incorrect conclusion. In a helicopter Induced Drag does not increase with speed it reduces. In a fixed wing aircraft as the aerofoil travels through the air it will induce a flow downwards, exactly the same happens in a helicopter. Imagine being in the hover, as each blade rotates its downwash is met immediately by the following blade. As a result, to create an angle of attack the pitch angle requires to be rigged much higher than a conventional aeroplane. So in the hover this induced flow is travelling directly downwards, the angle of attack is high and also the Induced Drag. Now imagine the helicopter beginning to move into forward flight. The moment the disk is tilted with the cyclic a further slight power increase will be needed to maintain height. But soon after, the rotor system will begin to experience an airflow from ahead. This horizontal airflow modifies the direction of the induced flow and in doing so increases the angle of attack (without a change is aerofoil pitch). This effect continues to occur with a continual progressive benefit until about 65kts (for the majority of helicopters) which is regarded as the minimum power speed. Understandably, this effect doesn't continue because as the helicopters speed increases so does the fuselage Parasite Drag and the rotor disk has to be tilted progressively further forward to provide an increasing horizontal component of Total Rotor Thrust to balance this increasing Parasite Drag. So as the speed increases above 65kts, Parasite Drag increase to V^3 up to the helicopters maximum speed.

The power to overcome parasite drag rises with v³, but the drag itself only with v².

Yes, thank you, that is an error in my text.


text:
Is this plasma-thruster experiment actually measuring thrust?
This experiment, which is being presented as an initial exploration of the feasibility of using microwave-generated plasmas in a sort of electic jet engine, has been attracting some attention in the general media, for example Ars Technica and Bloomberg News. To put it mildly, I am highly doubtful of the method used to estimate thrust, or even that it is demonstrating any thrust at all, but am I justified in that skepticism?
The experiment involves a vertical quartz-glass tube with an inside diameter of 24 mm. A compressor pumps air into the bottom, and immediately above that, the tube passes through the apparatus to ionize and heat the air, forming a plasma with a temperature up to 1,000 °C. The tube extends upwards beyond that for about another 23 cm, and the upper end is open to the air.

The thrust measuring device is a hollow steel sphere, diameter 75 mm, with a hole in it allowing small steel balls to be dropped into it to change its weight. In use, this sphere is balanced on the end of the quartz tube, and the weight is adjusted until the airflow causes it to wobble (there is a video in the figures section showing this.) The actual measurement of the "jet propulsion force" is the minimum weight of the sphere needed to prevent this wobbling.
On the basis of these measurements, the authors claim to have demonstrated, in this device, a "total propulsion pressure" of up to 24 kN/m², which they say is "comparable to those of a conventional jet engine of an airplane". I am not sure what "total propulsion pressure" is, but my back-of-envelope calculation suggests that they might be thinking of a jet engine's static thrust divided by the nozzle area. Regardless of whether that is so, their figure is simply the weight of the ball divided by the area of the tube, with the corresponding figure for the no-microwave situation subtracted.
Where does one begin to analyze this device? For one thing, at the point where the ball is not yet wobbling, there is presumably no flow (or just a little leakage.) Surely this is a static situation, with little or no relevance to what thrust might be generated? Furthermore, does the weight at which the ball will begin to wobble depend essentially entirely on how much pressure the compressor can reach with its outlet blocked? And pressure given here is only 1/4 atm. -- how much thrust can you hope for, from such a small expansion ratio, even with the best possible nozzle design and a 1,000 °C plasma?

Oh man, if that is science, I'm Scrooge McDuck. Like they didn't have any load cells in the lab

I do think this Q should be posted on physics SE...

@Jpe61: ...or Science Fiction & Fantasy SE.

This is an experimental device in a laboratory. Its connection to aviation is hypothetical, at best. This is really a question for [physics.se]

@Jpe61 I have no objection to this being migrated to the Physics SE, if that is possible. Having said that, I chose to post in Aviation because it looks to me more like an engineering issue than one of science, particularly as the 'buzz' created by this paper seems to be driven by the claim that this approach is already showing results comparable to current jet engines. I figured that an aero engineer, especially one with a specialization in propulsion, would be an authoritative source for a definitive answer. Some physicists who are not specialists in this field find it plausible.

This paper is quite obviously a troll. The ppl who published are definitely aiming for the Ig Nobel prize. I haven't checked, but I bet it has not been peer reviewed.

This is a decent question, but it's definitely one for Physics or Skeptics SE. The paper is full of holes. All I can say from the aero perspective is that they've basically invented an electric afterburner.

No, it's not measuring thrust.  It might be measuring a proxy for thrust density (force per unit area).  Anybody could use their setup to measure a ducted fan or a turbojet or even a rocket, but nobody does.  They themselves could have calibrated their rattling-ball instrument with a conventional thruster of known, conventionally measured, thrust (a $20 electric ducted fan from a hobby store).  (What's the threshold for rattling, anyways?)
As Jpe61 commented, it would have been much cheaper and simpler to place their thruster on a load cell (a fancy bathroom scale) and observe how downforce increases when the thruster is running.  $100 instead of fancy metalwork.  See, for example, fig. 6 and reference 23 of Prasad et al, "A laser-microfabricated electrohydrodynamic thruster for centimeter-scale aerial robots," https://arxiv.org/pdf/1906.10210.pdf.  See also the fancier inverted-pendulum thrust stand used at Princeton to measure plasma thrusters.

Come on Camille, you blatantly revealed what I was trying to mystify by using "load cell", I was going to write bathroom scale, but then I though not sciency enough, gotta upgrade.

I am being somewhat unfair here, as I did not specifically ask this in my original question, but I seriously doubt that the rattling ball could provide a valid indication of either thrust or thrust density, regardless of how it is calibrated, given that it significantly obstructs the flow. Would you consider that to be a reasonable objection to this experiment? At the very least, I think the validity should be justified in the paper, unless it were already a well-established technique (I am trying to give the authors every benefit of the doubt here.)

To be a valid scientific paper, it should have a hypothesis which is then studied using established methods. If this steelball -thingy is a valid established procedure, it will be stated in the paper. If not, well, you know. You can't do both at the same time, study new propulsion and measuring tech. It's either or.

There are really two things about this experiment, one is good science (if of dubious practicality), the other bad.
To take the bad first, the experimental setup is extremely crude and the thrust measurement/calculation inaccurate. There are far better ways to measure thrust. Another answer has commented on this. From this perspective, the experiment is worthless.
The good science is, perhaps surprisingly, the use of microwave heating to create thrust. The device is basically a thermal ramjet. You ram air into one end of a subtly asymmetric duct, heat it in the middle and allow it to expand out the back. A conventional ramjet burns fuel vapor to do the heating. Towards the end of WWII German researchers even had some experimental success with burning powdered coal and Lippisch (of Messerschmitt Komet rocket fighter fame) studied a delta-wing plane, the P.13, powered in this way. Rather less romantically, the Meredith principle uses the waste heat from an aero engine by ducting-in the radiator. First used on the Spitfire, it later boosted the Mustang's performance quite significantly. The US and USSR even tried nuclear reactors as the heat source! So why not use a microwave heater? A perfectly good idea in principle.
But, speaking as a sometime plasma power system engineer, making the thing efficient is a huge challenge. In my day it was all meaty inductors of coiled copper pipe with oil-based coolant flowing through, air- or vacuum-gap capacitors and huge inefficiencies (hence the oil cooling). No doubt we can nowadays do clever stuff with switched-mode solid-sate digitally-controlled games, but at the high voltages needed to strike and sustain a dense plasma in air, it would still be a big challenge to get the efficiency comparable to say an array of old-fashioned heating elements, where 100% of the "waste" heat is also the working heat.
One counter would be to raise the air temperature, and hence the thermodynamic efficiency, far higher than is possible with an incandescent wire. But that both makes the energy-density problem harder and creates nitrogen oxide emissions, which have to be strictly controlled.
There are other big engineering problems. With dubious efficiency, the power levels needed for aircraft flight would dump very large amounts of heat into the circuitry - keeping it all from melting would be hard. Also, a dense plasma has a high concentration of ions and free radicals (charged molecules), and hence is chemically highly reactive. It tends to eat one of the electrodes with unwelcome rapidity and, depending on the local conditions, will either eat into the duct walls or deposit oxidized electrode material on them. None of this is good for low maintenance or long-duration flights.
Finally, the thermal ramjet is inefficient at low speeds. It needs an alternative thruster to take off and get it up to a few hundred knots, and really only comes into its own at supersonic speeds. So it can never be a solution for subsonic mass air travel. A hypersonic suborbital trans-global spaceplane? Maybe one day, I won't bet on that one.
So - good science, dubious practicality.


text:
What’s this on the J-10 fighter?
What's this on the J-10 fighter?

Source: wikimedia.org

Related:  https://aviation.stackexchange.com/q/31799/60584

Air to air refueling probe.
Here is a youtube:

And for when that youtube goes away, a screencap of that:

That looks like an in-flight refueling probe.


text:
What is the purpose of the three small bumps on the switches for landing lights?
What is the purpose of the three small bumps on the switches for landing lights and what is the history behind it? You'll see a similar design on Boeing, Airbus, and other aircraft.
Here is a picture of a 777 overhead panel:

It's to make them distinguishable. See Is there a convention for type of switch used and where?

To help blind pilot finding the switches :)

The purpose is to make it distinguishable by touch or visual as explained in other answers and comments. And because generally, there are two main landing gear and a nose landing gear, it has been designed to be more like landing gears.
Edited:
There are two main questions about landing light switches in Aviation Regulation as it reads in FAR 23.1301 which is required to get AIRWORTHINESS COMPLIANCE certificate.

Is the landing light switch located so as to be readily
  accessible to the pilot?
Is the landing light switch adequately labeled as to operation
  and function performed?

It’s a tactile shaping so a pilot can recognize the switch just by feel.  Allows the aircrew to visually concentrate on more important tasks and simply locate and command the switch by touch alone.  Also useful if cockpit lighting is lost at night and you need to locate critical switches.


text:
How are aeronautical charts made?
How are aviation charts made? What kind of techniques are used?
From what I understand a GA aircraft equipped with:

A camera: takes images of the ground
Radar: calculates altitude and detects obstacles

I guess aviation charts are made out of printed paper. What are you exactly asking?

@HugoWoesthuis come on Hugo....

@Ben I didn't know jokes weren't allowed here;)

What kind of chart are you asking about?  Charts, as opposed to maps, provide information to support navigation.  Terrain and obstacle data is present on VFR Sectionals, but they also contain navigation and airspace information. IFR charts have little to no terrain data. Have you researched the AeroNav part of the FAA web site? It may help even if you're not in the US.

Like any other map, with GIS software.

That's a pretty broad question.  There's a lot of stuff that goes into an aeronautical chart.
From your "dot" items, I tend to think you asking how the ground elevation is determined, plus the elevation of obstacles of towers etc -- is that correct?

Mapping starts with basemaps, with topos (DEMs or digital elevation models) and geographic features.  Then cultural overlays are added.
The charts in US utilize a national spatial reference system. Today this is done with GIS/Mapping software. Then overlays for aviation data, such as man made obstructions, and air navigation data is added.
The (old) process has been the topic of many articles in pilot journals. The newer process is rapidly evolving, as the government transitions to AWS / cloud based applications.
To address your questions, the DEMs may have been created with RADAR and integrated with topo data, but today LIDAR is the norm.  LIDAR data helps with man made structures as well, while not as good as SAR RADAR, it can still show catenary and power line structures.  Those metallic structures show up very well in SAR due to their conductivity.


text:
Do Diesel engines have a mixture option?
Consider the DA-42, it has no mixture control. Is that because it is run on Jet-A/diesel? Or is the mixture control automated?

Related

@Pondlife, actually no, it isn't related. The Rotax engines are still spark-ignition, just with automatic mixture control, while here we talk about compression-ignition engine, where the principle is completely different.

@JanHudec I see what you mean. I was looking at it as a very general 'aircraft without mixture controls' topic, but you're right that it isn't a direct comparison.

For fuel, any fuel, to burn, it must be mixed with suitable amount of air. Neither too little nor too much would burn. Spark-ignition engines pre-mix the fuel with air, so they must do so in the right ratio, hence the need for mixture control (which might be automated, but it must be there).
However, compression-ignition (Diesel) engines are different. They inject fuel directly into the cylinders and as the fuel spreads from the injecting nozzle, it mixes with air and ignites at the moment it is sufficiently mixed. The cylinders always take full charge of air, otherwise the compression would not heat the air enough to ignite the fuel.
At low power, the flame is only small around the nozzle and at the large power it expands to fill the cylinder, but either way there is a point where the mixture is just right and the fuel ignites. Therefore compression-ignition engines don't need mixture control. The power lever simply controls the fuel flow into the engine and there is no throttle on the air intake (so calling power lever ‘throttle’ is inaccurate for Diesels (and turbines)).

In your first para: "Compression-ignition engines pre-mix the fuel with air" shouldn't that be "Spark-ignition engines..."?

@FreeMan, thanks. Of course I meant to say ‘spark-ignition’ there.

Assumed so, but you know what happens when you assume... :)

In a gas engine, the fuel is mixed with air, usually in a carburetor, before entering the cylinders, but there is no such pre-mixing at all in the diesel, where a metered amount of fuel is injected directly into the combustion chamber, in the midst of very hot, highly compressed air. Thus, the diesel does always work with an excess of air.

I don't think the diesel is injected into the compressed air. IIRC, it happens earlier in the cycle. This gives the diesel time to disperse. The mixture is then compressed and ignites everywhere.

@MSalters What you describe is not a Diesel, but an oil engine of the Hornsby-Akroyd type, later known as hot-bulb engines. Sometimes, they are (improperly) called 'semi-diesels'. They were much used in agricultural machinery and in small fishing boats.

No, the "ignites everywhere" specifically excludes hot bulb engines. Compression ignition (which is characteristic for diesels) is how you achieve that.


text:
Why are holding patterns in an oval shape?
Why are holding patterns in an oval shape? Why not circular or more square-like?

Not all holdings are. There are circular holdings. Mostly for VFR traffic, though.

Airplanes can't make a perfect 90 degree turn

I don't have any definitive information on why the oval shape was chosen. However, it seems to me to be the most practical shape given the navigational equipment available when holding patterns first became necessary. VOR, DME, and GPS technology did not exist. Holding patterns were flown, often by single pilots hand flying, with reference to an ADF needle, maybe with a little marker beacon help.
I don't know if a square-like pattern was considered, but if it was I think it would have been rejected for at least the following;

a rectangular pattern would involve 4 turns, better to keep it simple with only 2 turns.
flying any kind of a leg perpendicular to the holding radial would mean getting farther away from the radial than necessary with no advantage to be gained, and it might involve more terrain clearance problems.

Flying a circular pattern makes it more difficult to keep track of where you are in your head (no glass cockpits, indeed no HSIs even, not even rotating ADF cards at first), than an oval. Worse, though, is what could occur in some wind conditions. For example, let's say you're supposed to hold north of a fix on the 360 radial (inbound course 180). Further, lets say there's a significant north wind. You're approaching the fix on a course of 180. You cross the fix and then fly a circle with respect to the air you're in. In a no-wind condition, you'll come back to the fix. However, with a north wind, you'll come back to the course south of the fix, and will have thus violated where you were supposed to hold.
With today's navigational technology, we can of course easily fly any kind of shape we want, but the disadvantages previously mentioned would still apply.

An IFR holding pattern is along some bearing (typically, a VOR radial).
By flying a racetrack pattern, the plane is flying inbound on the radial for a period of time (straight line), making a standard turn (semi-circle), flying outbound on the same heading, slightly offset from the radial (straight line again), and making another standard turn to come inbound again.
On each pass, the pilot has time to verify his heading and alignment on the radial, and correct as needed.
If instead he were flying a circle, he'd never be established on any fixed heading for more than a moment.  His heading would always be turning, which is a chance to get disoriented, uncertain, and off course.
A constant circle would also be less comfortable for passengers.  With a standard hold, they get a couple minutes of straight flight before a gentle turn.
Imagine trying to turn a perfect circle, at night, in clouds, in stormy, turbulent weather.  Without GPS, you could never be sure you were in the right spot. (aviation techniques were created long before the days of GPS)

According to various sources I've seen, the original purpose of the 'racetrack' holding pattern was to eliminate cumulative precessional error in gyroscopic intruments such as simple attitude indicators. Basically, the straight legs of the track allow the gyros to settle down again after a turn, and (just as importantly) allow the pilot to check that the instruments have settled down again, before starting the next 180° turn.
As was well explained in this answer to another question, in a good gyro there should not actually be any cumulative error in continuous turns because, at each point in a 360° turn, the error introduced by the gyro's 'erecting mechanism' should be precisely cancelled 180° later. But, in early instruments there certainly were cumulative errors (eg. due to frictional forces on the gimbal), and the racetrack pattern was a simple answer to that problem.

Wind correction in a "parallel" course racetrack pattern is much less complicated than in a circular pattern. This is the number one reason.
Square patterns would require steep turns, inconsistent with the concept of a standard rate turn in instrument flight.
It's not about comfort, it is about navigation and maintaining the aircraft reliably in a specified region.


text:
Why do some aircraft have pods on their wing tips?
By JetPix [GFDL 1.2 or GFDL 1.2], via Wikimedia Commons
Why do some aircraft have pods on their wing tips, like this Cessna 414?

Wingtip pods or winglets differ in installation reason from aircraft to aircraft. Here is a non exhaustive list of possible reasons:

Fuel tanks
Whitcomb bodies to enhance the area rule of the aircraft also known as Kuchemann Carrots
Vortex generators and relievers - used to enhance aircraft aerodynamics and modify fuel consumption
Mass balance - in particular on forward swept wing aircraft
Housings for sensors or landing gear

Many aircraft have had fuel tanks mounted on the wing tips. These were sometimes main tanks, but often were auxiliary tanks, or even "Drop Tanks" which could be jettisoned for combat.
The tip tanks on the Cessna 414 pictured above are the MAIN fuel tanks. They hold 50 US gals each. In addition to the 50 gal mains there would be 2 smaller auxiliary fuel tanks in the wings, and 2 optional "wing locker" fuel tanks in the wing lockers behind the engines.
All early Cessna 300 and 400 series aircraft shared this rather complex fuel system which required using fuel from the 50 gal mains first before using the auxiliary or wing locker tanks.
Later versions of the Cessna 400 series aircraft went to a much simpler "Wet Wing" fuel tank system with only 1 tank in each wing.

Another example of an aircraft with wingtip tanks is the T2-C Buckeye. They hold approximately 102 gallons each, and so add to the total fuel capacity of the aircraft.
There are a number of reasons that the wingtip tanks might be part of the design of an aircraft.
Decreasing flutter is one reason that you might add wing tips, and this  hasn't been mentioned in any previous post. Reducing flutter will decrease drag, and that offset the increased drag of adding external tanks.
I think one of the things that made the T2-Buckeye ideal as a trainer is that the wing tips reduce roll rate. That is good for training but not so good for bombing or aerial combat.
It also reduces airflow escaping off the tip which creates vortices.
The wingtip tanks also help in the distribution of weight, which effects the center of gravity. Weight at the end of the wing will offset the bending of the wing due to lift.

It was the only jet aircraft that the Navy permitted spins in, and it was therefore used for training in spin recovery procedures.
This was the US Navy's basic jet trainer. It was also the only jet that you were allowed to intentionally spin. That was a lot of fun. It had a very tight turn radius. It was where a pilot got their first hop in formation flying, rendezvousing, and the gunnery pattern. Carrier qualifications were done here as well, although one didn't always get to trap aboard a carrier. First time a pilot did a lot of consecutive solos as well. Very cool flight time in the training program here.


text:
How do bearings in a jet engine keep water out when it's been cleaned?
When a jet engine is running, it's generally immune to any liquid in the incoming air as the liquid would have been evaporated in the compressor. However, when the engine is being cleaned for carbon deposits, cleaning solution mist is injected is fed to the inlet, and the water would have the chance of entering the compressor bleed. Is there any special design in the bleed path to keep the water out, or the water simply couldn't get past the labyrinthine seals around the bearings?

"the liquid would have been evaporated in the compressor" uhm, considering we are talking about adiabatic compression, that's not a given. you can liquefy gases by compressing them. sure, the temperature rise would work against that.

How do your car's wheel bearings keep water out when you drive in the  rain?

Bearings are not exposed to air, they need to be lubricated and oil has to be in a closed space (sump), see How does a labyrinth seal work when the engine isn't rotating?

@jamesqf Car bearings are self-contained and do not circulate its lubricant in an open loop or intentionally mix its precious lubricant with ambient air, nor do car bearings rotate at 10,000 rpm.

To keep dirt out, bearings with exposed faces have rotary seals on them. Those seals will also keep water out, as long as the bearing is not rotating when wetted and not completely immersed. If immersed for days or weeks, even well-sealed bearings will eventually allow water in.

Water is kept out because the oil is supplied to the bearings at a higher pressure than the surrounding air, so the oil mist is blowing away the air, not the other way around.


text:
Is the Meredith effect utilized in the cowling design of air-cooled engines?
The P-51 is famous for its advanced-for-its-time aerodynamics, the prime example of which is the radiator design under the belly which utilizes the Meredith effect to, in effect, turn it into a mini jet engine. Now, the cylinder walls and cooling fins of air-cooled radials are much hotter than whatever cooling liquid that's used in the P-51's water cooler, so is it possible to produce a similar cowling for the air-cooled radials? Is there any example of such a cowling on WWII airplanes?

The Meredith effect is to recover radiator drag by using engine heat for thrust, just like a jet engine.  It worked very well in the P-51 Mustang, actually adding a few knots of airspeed.
It also would work well with any source of heat, including a nuclear reactor, so yes it would work with aircooled engines.
The issue is drag from the radiator.  Aircooled radials have much greater frontal drag, so it worked, just not as well.  Efforts to reduce drag in the B-36 bomber resulted in roasted engines, particularly the rearward banks of the "corncob" R-4360 radial.  Overheating is a major concern, as is drag.  Engineers try to find a good balance.

the way I see it if we stay away from crazies like the Wasp Major the problem to solve is how to recover the pressure from the leeward half of the cylinder. since it can not be made like a golf ball some sort of guide vane must be present at the leeward side lest the turbulent wake forms before the second row of cylinders.

Added to the turn to jets.  Ton mile/gallon favors higher speed and less drag, even with slightly less efficient propulsion.  Amazing that the freight train, with all cars following the first, is 4x more fuel efficient than my tractor trailer.  The 747 is almost 2x more efficient than props, even though piston engines get better miles per gallon.

The NACA cowling comes to mind

I had to go look up what the "Meredith Effect" was (and edited your question to include a link to Wikipedia).
As I was reading through the Wiki article, I found the answer to your question:

Many engineers did not understand the operating principles of the effect. A common mistake was the idea that the air-cooled radial engine would benefit most, because its fins ran hotter than the radiator of a liquid-cooled engine, with the mistake persisting even as late as 1949.
  (Source)

So no, the Meredith Effect will not work on an air-cooled engine.

please read your own quote "does not benefit MOST". It does NOT say "does not enefit AT ALL". The Meredith Effect is not some inexplicable high science, an inlet does pressure recovery, the radiator adds heat to air, a nozzle reconverts pressure back to velocity. all I'm asking is whether someone had BOTHERED to design a cowling to make a REAL EFFORT at utilizing it rather than just be SATISFIED that the engine is cooled and not TOO MUCH DRAG is induced.

@MeatballPrincess There is no need to yell, you can use markup in comments to denote emphasis

@AEhere Honestly, boldface looks just as shouty as all-caps, to me.

Your quote states it won’t work very well, but it unfortunately doesn’t state why. I think the other answer hits this nail on the head; the radiator flow exits nicely into the airstream, as it is aligned with the fuselage, and doesn’t have any obstacles in the exit flow. In comparison, the flow around the cylinder heads of a radial engine, aren’t smoothly aligned with any surface, and lots of other brackets, piping, and whatever on the outside of the engine create turbulence and cause a loss in the total pressure of the cooling flow, and hence the jet thrust.


text:
Are turbofans compensated for gyro effect?
In a multi-spool design is the gyro effect canceled or mitigated by having the two or more spools spinning in different directions?

Related: Why does the LP compressor rotate in the opposite direction of the HP compressor? and Why are contra-rotating jet engines so rare?

There are a few counter rotating turbo fans, such as the GEnx. But the majority are co rotating.


text:
How do airliners with engines mounted on places other than wings suppress wing flutter?
Assuming airliners typically have high aspect ratio flexible wings, how are wing flutter suppressed when engines are not available on forward-reaching pylons as weights?

Since flutter is a natural-frequency harmonics phenomenon, it's a matter of "tuning" the wing box structure with the required stiffness and damping to move its natural frequency range outside the range experienced with its C of G and torsional center offset and uncompensated for with engines on pylons.
It's a bit like with flight controls.  With manual ailerons, the aileron/wing system is flexible you could say, and needs mass balancing.  Like wing mounted engines.
With hydraulic controls, the connection between the aileron and wing is rigid outside of movement induced by the hydraulic actuator.  No mass balance required.  Like a wing box with no engines that has to be inherently stiffer than otherwise.
It's one reason wing mounted engines are more common than tail mounted.  Lets you make the wing box structure a little lighter.


text:
How are these parameters derived from readings of sensors in an aircraft: angle of attack, slip angle, and airspeed?
From what I've learned, the reading comes from

an angle of attack sensor and a slip angle sensor. Let's call the readings $\hat\alpha$ and $\hat\beta$ respectively. These are of either the weathervane type or the differential pressure type.
airspeed sensor, i.e. a pitot tube. This probe measures the total and static pressure $\hat p_\text{total}$ and $\hat p_\text{static}$, and the airspeed $\hat u$ is obtained from them.

Here's the catch: in real flight, the measured angles $\hat\alpha$ and $\hat\beta$ may depend on not only the true values $\alpha$ and $\beta$ but also other factors which include true airspeed $u$, and the measured airspeed $\hat u$ may depend on $\alpha$ and $\beta$ too. How is this problem solved? How are these sensors calibrated? Are they tested in a wind tunnel where every possible combination of $(\alpha,\beta,u)$ is given to produce $(\hat\alpha,\hat\beta,\hat u)$ and form a lookup table?

Here's a fun fact. Nothing you measure is what you intend to measure. The corrections of these measurements to satisfy the objectives form a good chunk of development flight test by the airplane manufacturer. Even after the airplane type is certified and enters into service, the corrections may continue!
Now specifically for AOA ($\alpha$) and AOS ($\beta$). In development flight test, an accurate measurement of AOA and AOS is typically done with a nose-boom that places it outside of the aircraft boundary layer (as reasonably can). But there's an error even with that, due to Mach, manufacturing tolerance, etc. Flight path reconstruction could be performed to further decrease the errors, if needed. Usually, the boom AOA and AOS measurements are pretty close to the actual AOA defined relative to the body axis and used in wind tunnel.
In service, AOA is measured with vanes located at the nose of the aircraft. Even at low speed where Mach effect is small, these measurements are not the aircraft AOA. They are local AOA due to upwash and boundary layer. For non FBW aircraft, AOA measurement is used primarily for stall warning and stall protection (SPS). There are lookup tables in the SPS that register the stick shaker and pusher (if required) angles, as a function of the local AOAs. An extra mapping, as a function of Mach, configuration, whatever, is required to transform the local AOAs to aircraft AOA, should the design need it.
Development flight test airspeed is usually measured by the nose-boom, often augmented by an additional trailing cone. The trailing cone is extended behind the aircraft to measure the far field static pressure. Total pressure is measured by the boom. The resulting impact pressure needs to be transformed to dynamic pressure to output airspeed.
In service airspeed is measured most often by pitot-static tubes at the nose. Due to boundary layer, static source error correction (SSEC) would be required. These lookup tables are stored in the air data system (ADS). There may also be a mixing  of the left/right static ports to decrease the effect of asymmetry.

They use test flights.
They attach more instruments to the plane and move them outside the influence of the planes own turbulence.
From the known good values of these test instruments they can calibrate the actual instruments.

The final calibration is done through test flights, and before the first flight the calibration is based on CFD, and possibly wind tunnel tests (depending on the complexity and the budget of the aircraft under development). You may want to expand the answer.

When the manufacturer builds a plane, they make a table with correction of readings according to altitude and position of the sensor


text:
At what range does the turbofan start to become more economical than a turboprop?
By more economical I only mean 'burns less fuel'. I'm under the impression that at optimal cruise speed and altitude a turbofan will burn less fuel per mile per passenger than a turboprop, but I might be wrong. I'm also under the impression that this doesn't mean turbofans burn less fuel for every mission, that over shorter distances the time taken to reach the optimal cruise doesn't allow for the efficiency of turbofans to come into effect.
Also, unrelated to the efficiency of the aircraft, does the speed of jet aircraft play a significant role in the revenue of airlines? Does the speed increase of turbofans over turboprops mean more flights, or is this time saving not of great significance when designing and selecting aircraft?

So is the question about range or speed? You can edit it by clicking the edit button.

Does this answer your question? Which engine is more efficient between turboprop vs jet? From this answer: "The break even point is somewhere between 300 and 500 NM."

@vasin1987 The second paragraph is about speed, the first is a more complex question that I may have summarised too quickly, it's not about speed or range.

@Bianfable Yes it does!  Thanks, I need to do some more research to determine which factors this depends on.


text:
Where do these limitations on wingspan come from?

There may be a limitation on the maximum value of wing span in some
cases, for example naval aircraft. Airlines have to be able to operate
into airport gates of limited width and typical values of span for
different classes of transport aircraft are given in Table 5.2.

Aircraft Conceptual Design Synthesis by Denis Howe
I thought this might be Aeroplane Design Groups, but the values don't match up. Does anyone know where these values come from? Also, how does the B757 get away with a greater wingspan?

The text says the values in that chart are "typical," not regulatory.  Perhaps there is less authority to the values there than it appears; the B-737 wingspan is 117', and it along with the A-320 pretty well define short/medium haul airliner, though it'd be too big for that category according to that table.

Are you asking where the actual wingspan figures in table 5.2 come from, or why there are limitations?

The B757 was designed as a variant of the B767, so for many purposes it needs to be treated as a wide/heavy plane too even though it may not technically qualify.

@MichaelHall I was looking for some regulation or convention that I could point to. I'm designing a short-haul aircraft for an assignment. Currently, my wing is a little longer than the short haul requirement (given AR and S). But it fits comfortably within an ICAO and FAA category. If I want it to be suited for short-haul as per Howe's recommendations I'd have to trim my wingspan with winglets or lower the aspect ratio.

Two points appear relevant here.
First, all aircraft designers have exactly the same laws of physics to work against, and the same rules of materials science to work with, and those considerations also define the design of the engines used to power their aircraft.
This means that an Airbus is going to closely resemble a Boeing that was designed for the same stage lengths and payload, right down to the fuselage lengths and length-to-diameter ratio, wingspan, wing aspect ratio, sweep angle, overall dry weight, useful load, fuel capacity and engine thrust.
Second, it then becomes the responsibility of the airport terminal designer to accommodate those fuselage lengths, wingspans, and tire loads on the pavement in such a manner as to prevent interference between adjacent planes while being maneuvered into position at their gates and prevent the pavement from cracking under the tire loads.
Since a big airport has to accommodate a variety of plane sizes, the airport designer has to position the gates to accommodate not just a 737 but almost everything else- and the optimum design is one in which the biggest planes get handled by gates at the convexities of the terminal and the smallest ones are serviced by the ones in the resulting concavities.


text:
Is there a good way to find out or estimate the prices of certain engines?
I'm specifically looking for the price of a PW150 (5070bhp, Turboprop) and some 55kN Turbofan engine. I'm doing some Direct Operating Cost analysis but I have no idea how much these components should cost.

With so little information, no, there isn't. There are many other factors, such as qty, new vs used, specific model, maintenance agreement, etc.

Have you tried contacting the manufacturer?  That would be the first place I would go for a quote...


text:
How does the maintenance cost of turboprops compare to turbofans?
I was initially under the impression that the added complexity of gearing in a turboprop would lead to more maintenance. Then I found out geared turbofans are becoming fairly common so there should be no difference in maintenance on that account.
I also thought the bypass air would play a role in cooling the casing for the combustion chamber onwards, but maybe this isn't a relevant factor.
Does anyone know how the maintenance cost of these two types of engines compare?


text:
Are shockwaves responsible for heating in very sparse atmospheres?
I recall learning about hypersonic flow in high atmosphere. I think one of the modelling assumptions used is that the relative air flow is so fast, and the average space between molecules is so great, that you can treat each molecule as individually hitting air/spacecraft surfaces and imparting momentum.
Question 1 is if my memory of this assumption is correct?
Question 2, if this is the case then do shockwaves form only when the air is sufficiently dense?
Question 3, is it shockwaves that are responsible for heating (of flow) in this scenario, or simply molecules colliding with an air/spacecraft?

you can treat each molecule as individually hitting air/spacecraft surfaces and imparting momentum.

This is true for satellites in low orbits, but not for airplanes which get their lift from conventional wings. This kind of flow is called rarefied as opposed to continuous flow for aerodynamic lift.

do shockwaves form only when the air is sufficiently dense?

Yes and no. Shockwaves are happening in continuous flow, but the pattern of energy and speed changes of molecules in rarefied flow are very similar to those in continuous flow, except that the energy at orbital speeds will cause ionization and complex chemical reactions. Note that Newton's impact theory, while not suitable for slower flows, starts to become a good approximation for hypersonic flow. Note as well, though, that concepts based in statistic means like pressure and temperature do not apply to rarefied flows.

is it shockwaves that are responsible for heating (of flow) in this scenario, or simply molecules colliding with an air/spacecraft?

The change in heat through a shock wave is caused by the rise in pressure and, especially in the steep middle of a detached shock, by friction within the shock. The skin temperature of a hypersonic airplane is caused by friction as well as heat transfer between air and skin. Since the direction of the flow is changed by an oblique shock, air molecules do not simply bump into the airplane but are redirected by the surrounding pressure field. Now you might say that they now bump into the pressure field, but that is not like an impact between solids. Instead, many numbers of collisions between molecules make them all change speed and direction. Of course the pressure field around an airplane is in turn the result of zillions of individual impacts between airplane and air molecules.

Imparting momentum (actually the aircraft hits the air) in relative sense can be described by molecules hitting the aircraft, but this is more aptly described as mv/s, aka drag.
Heating occurs when energy from impact of individual air molecules going very fast is enough to increase the rate of movement of molecules on the surface of the aircraft.  What we know as "heat" is actually a measure of how fast the molecules of a solid are moving, and if sufficient energy is applied, the solid melts or even vaporizes.
Heating occurs at very high altitudes where the True Airspeed is extremely fast, as in multiple Mach.  This was referred to in the early days as the "thermal" barrier, beyond the sound barrier.
If a shock wave builds in front of the aircraft, then the collisions will be absorbed by it, with the potential of heating it to an ionized plasma, literally breaking the air molecules into atoms.  This phenomena actually lead to a miscalculation of the center of lift during the first Space Shuttle re-entry.  The resulting nose-up had to be corrected by dropping a flap on the rear of the delta wing.
To summarize:  heating is the result of high energy collisions that input energy faster than it can dissipate.

It's a little like throwing 1000 feather pillows at a bell compared to shooting a bb pellet.  The "ring" is vibration, analogous to heat.  1000 pillows will make it sway, the bb makes it ring.


text:
What is the benefit of a horn control surface?

Like the one above. Is it just so that you can hinge closer to the leading edge of the aileron or control surface? Why did the designers of the ATR-72 elect to use this feature?

There are two coefficients which determine the forces needed to deflect a control surface (besides the physical parameters like area and dynamic pressure):

Change in control surface hinge moment coefficient $c_r$ with angle of attack $\alpha$ : $c_{r\alpha}=\frac{\delta c_r}{\delta\alpha}$
Change in control surface hinge moment coefficient $c_r$ with control surface deflection $\eta$ : $c_{r\eta}=\frac{\delta c_r}{\delta\eta}$

By shifting the hinge line back, an overhanging nose of the control surface is created which reduces hinge moments because the forces here will counteract the forces on the rear part of the control surface. This overhanging nose will reduce both coefficients equally.
In contrast to that, the horn, while also creating counteracting forces, will affect $c_{r\alpha}$ more than $c_{r\eta}$. This allows the airplane designer to tailor the ratio between both coefficients. Why this is important becomes clear once you look at the floating angle of a control surface:
$$\eta_f = -\frac{c_{r0}+c_{r\alpha}\cdot\alpha}{c_{r\eta}}$$
Normally, both coefficients are negative and the absolute value of $c_{r\eta}$ is a bit higher than that of $c_{r\alpha}$. And that is good: A positive $c_{r\eta}$ means that propelling hinge moments will increase with flap deflection, making the flap unstable and run into its stops immediately. But a positive $c_{r\alpha}$ just means that the control surface will deflect into the airstream when angle of attack changes, increasing stick-free stability over stick-fixed stability. So it is desirable to make $c_{r\alpha}$ very small while keeping $c_{r\eta}$ decidedly negative. This can be accomplished with a horn on the control surface.

Because the horn extends ahead of the hinge line, the slipstream tends to pull  the rudder deeper into deflection. This reduces the amount of control force needed to work the rudder without servo-boosting it. This is called aerodynamic boost and can be used to boost the ailerons and the elevator as well.


text:
Is it legal for pilots to transport or use marijuana?
More and more states are legalizing the recreational use of marijuana. What does that mean for pilots?

Is it legal for pilots to transport marijuana within states that have legalized it?
Can pilots use marijuana recreationally if their state allows it? Are there implications for medicals?

Don't know.  2. Probably need some hours of 'clean' before flying, like '8 hours, bottle to throtte" for alchohol.  Except there's no test to see how high someone is that I am aware of, and people metabolize(?) it at different rates.  Medicals now don't test for drunkenness or being on other drugs - would red eyes be a giveaway for having been stoned recently?

@CrossRoads It is not allowed legally to fly with it (see my answer). Follicle tests can detect MJ use up to 90 days after consumption. Also (not that people don't), lying on the medical about use of MJ (or any other lie) is actually a crime (imprisonment up to 5 years and up to $250,000 in fines). Medicals sort of do test for that (with your AME or if on BasicMed, your doctors interview and the form you fill out).

§91.19 Carriage of narcotic drugs, marihuana, and depressant or stimulant drugs or substances.
(a) Except as provided in paragraph (b) of this section, no person may operate a civil aircraft within the United States with knowledge that narcotic drugs, marihuana, and depressant or stimulant drugs or substances as defined in Federal or State statutes are carried in the aircraft.
(b) Paragraph (a) of this section does not apply to any carriage of narcotic drugs, marihuana, and depressant or stimulant drugs or substances authorized by or under any Federal or State statute or by any Federal or State agency.

This gets into the classic State versus Federal laws argument. On one hand, if a "permissive" state agency finds it, they will not have any cause to report it. If a non-permissive state agency finds it, they can take action, (fines and jail time) and in addition they can report it to the Feds.
If the Feds find it (e.g. customs inspection), or it was reported to them via a state agency; the Federal government's position is that this is a violation and can (and have) take steps to remove your licence.

(b) seems to say carriage of drugs is legal if authorized by/under any "State statute" or "State agency".  One could argue that's the case in states that have legalized marijuana, medically and/or recreationally, regardless of who finds it.  But I wouldn't want to be the test case for that...

there is a doctrine of Federal Preemption
Under the doctrine of preemption, which is based on the Supremacy Clause, federal law preempts state law, even when the laws conflict. Thus, a federal court may require a state to stop certain behavior it believes interferes with, or is in conflict with, federal law. link
also keep in mind the fact that the state did not declare it legal, it simply removed the laws declaring it illegal

Is it legal for pilots to transport marijuana within states that have legalized it?

This I'm not sure about. I can't find any FAA regulation or guideline on it, so I think it would depend more on specific state statutes. The problem would lie in going over state lines, obviously, but intra-state transport is unclear. If you were ramp-checked by an FAA inspector and had marijuana on board, they would probably require you to get a drug test, and if failed (according to the next part of this question) you would lose your license (actually your medical, you would keep the license but be unable to use it). So it may be legal to transport it, but you certainly can't be the one using it.

Can pilots use marijuana recreationally if their state allows it? Are there implications for medicals?

Negative. AME's have marijuana on the DNF/DNI (do not fly/do not issue) list:

Controlled Substances (Schedules I - V). An open prescription for chronic or intermittent use of any drug or substance.
  This includes medical marijuana, even if legally allowed or prescribed under state law.

While the guidance just says medical marijuana, I would suppose that it includes legal recreational marijuana as well.
Since you would be using a controlled substance on the DNF list, you would not be eligible for flying under the BasicMed either:

A diagnosis or medical history of substance dependence is disqualifying unless there is established clinical evidence, satisfactory to the Federal Air Surgeon, of recovery, including sustained total abstinence from the substance(s) for not less than the preceding 2 years. A history of substance abuse within the preceding 2 years is disqualifying. Substance includes alcohol and other drugs (i.e., PCP, sedatives and hypnotics, anxiolytics, marijuana, cocaine, opioids, amphetamines, hallucinogens, and other psychoactive drugs or chemicals)

Another way to look at it: Since pilots licenses are issued by a Federal authority, the Federal rules apply. According to Federal laws, marijuana is illegal and your license (or medical) is not valid even if the state you are in allows it legally. Marijuana is still a Schedule 1 substance (on the same level Federally with Heroin and LSD).

AOPA has a nice article on it here according to them even if legal you cant transport it.

As a Schedule I drug, it is treated as illegal under the FARs
  regardless of whether recreational or medical use is permitted by
  state law. Several laws and regulations address airman use,
  possession, and/or carriage of marijuana; the FAA further addresses it
  in its medical standards.

If you use it, even legally or accidentally ingest THC, and the FAA finds out they will pull your medical as was the case for this Colorado based pilot.

According to this article since the aircraft is in federal airspace its even illegal for passengers to transport it.

Can an aircraft passenger bring legalized marijuana along on a trip,
  say between two states that have approved legalization? How do pilots,
  charter companies, and airlines deal with the issue?
The answer is fairly clear. All public airports, airspace, and
  aircraft are governed by federal law and subject to federal law
  enforcement. Therefore, a passenger may never legally fly on an
  aircraft with marijuana or products containing marijuana. This is true
  even if you have a medical marijuana identification card and even if
  you are flying between two locations in the same state that legalizes
  the use. By entering the airport, you are entering a federal
  jurisdiction and from a federal perspective, there is no exception.